The state of the art includes roofs employed as a removable covering element in electric arc furnaces and in ladle furnaces so as to prevent the dispersion of heat from inside the furnace and the leakage of noxious fumes and volatile waste.
These roofs normally include at least a central aperture for the electrodes and a peripheral aperture, or fourth hole, connected to an intake system, to discharge the fumes and the volatile particles of waste and powders from inside the melting volume.
To avoid an excessive heating of the roof, and to remove the heat generated during the melting cycles, the roofs are equipped with a cooling system consisting of a plurality of cooling conduits, usually closely adjacent to each other, fed with cooling fluid under pressure and having a radial development, or circular with rings, or helical, or coiled or some other form and achieving heat exchange surfaces which may be vertical, horizontal or sloping.
One problem in the embodiment of such cooling systems is that the temperatures are distributed in a non-homogeneous manner over the surface of the roof.
In fact, it is well known that the heat in the central part of the roof, where the electrodes are, is greater than that in the peripheral part due to the heat irradiance caused by the electrodes themselves. Moreover, the temperature near the aperture through which the fumes are discharged is greater than that on the opposite side because of the greater flow of hot fumes towards this aperture.
In point of fact, the discharge aperture is associated with aspiration systems which create a non-uniform depression inside the furnace.
This aspiration creates a preferential flow of fumes and especially of the air entering the melting volume through the technological apertures (slag door, apertures for the burners, lances, to introduce additives, etc.) and also through the imperfect seals on the mechanical connections, generating a lack of thermal homogeneity inside the melting volume.
To be more exact, the imperfect sealing of the roof and the side wall of the furnace, together with the negative pressure which is created in the zone below the roof due to the aspiration caused by the fume discharge systems, causes an influx of atmospheric air which comes into contact with the electrodes.
Because of this phenomenon, there is an extremely high consumption of the graphite of which the electrodes consist, due to the fact that the oxygen contained in the air, as it laps the extremely hot lateral surface, causes the graphite to oxidise.
A further technological shortcoming is that the air, as it passes through the area occupied by the melting volume, causes thermal imbalance, cooling the whole area and not allowing a favourable operation in energy terms.
Finally, especially in ladle furnaces, and especially during the refining step, it is highly desirable, for economic and technological reasons, to obtain a condition whereby the atmosphere above the melting volume is controlled in order to obtain, quickly and accurately, a desired composition for the steel to be produced.
These conditions are not achieved in embodiments known to the art, wherein the air which inevitably enters the melting volume through the circular separating interspace between the roof and the shell of the furnace--since it is impossible to seal the melting volume permanently and perfectly--comes into direct contact with the atmosphere created inside the melting volume.
The influx of atmospheric air into the melting volume causes an oxidation effect on the slag and the liquid metal, which in turn leads to a worsening in the quality of liquid metal produced.
The non-homogeneous depression which is thus created inside the furnace, moreover, causes a high consumption--or even the removal--of binding materials, technological materials and slag-forming materials which are carried away in the main flow.
It is therefore necessary to reduce this removal, with the aim of recovering the technological powders and materials, by causing the fumes to flow at limited speeds, and therefore reducing the capacity which the fumes have of carrying away the fine volatile elements which are suspended in the melting volume.
This is to have an immediate economic advantage deriving from a greater yield of the materials loaded into the furnace.
Another disadvantage of cooled roofs such as are known to the art is that they cause heat to be removed in a substantially uniform manner over the whole of their surface. This means that the removal of heat must be at least equal to that required at the hottest zone of the furnace and therefore, for a large part of the surface of the roof, the cooling system is over-sized, which causes high energy consumption and greater costs of the plant.
There are also roofs in which, in order to extend their working life, the cooling conduits are protected, at least on the side facing the inside of the furnace, by refractory material.
This embodiment, however, has not been very efficacious since the refractory material wears very quickly, and then tends to become detached and to fall inside the furnace, due to the considerable differences in temperature between the inner face of the refractory material facing the electrodes and the outer face of the refractory material which is in contact with the cooling conduits.
There are also roofs consisting of cooling panels, for example with a conformation of contiguously arranged segments. Each panel has a group of cooling conduits associated with an individual cooling system or a common cooling system.
In the first case, the construction and management costs are very high, while in the second case the welds between the individual panels, or even between the individual elements of the cooling conduits, constitute critical points and create tensions along the conduit which cannot be completely eliminated even by heat treatments such as tempering or annealing.
These tensions, due to the conditions of extremely high temperatures to which the tubes are subjected, may lead to the welds breaking, with the resulting leakage of cooling water which will enter the furnace. Given the high pressure of the water which is usually circulating in the cooling conduits, there is a considerable quantity of water which penetrates into the furnace, and as soon as it comes into contact with the molten metal it immediately evaporates; this leads to a sudden increase in pressure which may, in certain conditions, lead to an explosion.
If this phenomenon should occur, the furnace must be switched off immediately, with all the technical and economic problems which that entails, apart from the potential danger for the workers.
There are also roofs equipped with shower-type cooling systems, using jets of water cooperating with the outer surface of the roof.
The advantage of shower-type cooling systems is that it is possible to distribute the jets of water over the surface of the roof as one wants, in such a way as to obtain a greater cooling effect in the hottest zones. On the other hand, however, shower-type cooling systems have the disadvantage that the removal of heat in the peripheral zone of the roof, where the sprayed cooling water is collected, is very high even though in this peripheral zone less heat should be removed than from the hotter, central zone.
In none of the embodiments such as are known to the art has there ever been proposed a solution which makes it possible to act on every area of the cooling system of the roof of the melting volume in such a way as to make the thermal loads uniform, to achieve the desired and appropriate conditions of controlled atmosphere inside the melting volume, to avoid unbalancing the technological operation of melting and refining due to the mixing of air with the fumes from the melting process, and to drastically diminish the consumption of the graphite in the electrodes.
Documents U.S. Pat. No. 4,401,464 and SU-A-1759894 propose solutions which provide to create a zone of depression immediately under the roof to prevent the atmospheric air which enters through the technological apertures from affecting the liquid metal inside the furnace.
These embodiments however do not prevent the influx of atmospheric air from coming into contact with the electrodes, with the resulting problems of premature wear on the electrodes.
Moreover, these embodiments do not achieve efficient operations to separate the powders and to recover and re-use the volatile slag mixed in with the fumes which are discharged from the furnace.
A further problem of embodiments known to the art concerns the wear and deterioration of the mechanical characteristics of the electrode-bearing cover arranged at the center of the roof.
This wear involves greater maintenance costs and long intervention times.
The present Applicant has designed, tested and embodied this invention to overcome the shortcomings of the state of the art and to obtain further advantages as shown hereinafter.